Calling the investigation into his hiring of family members for fake parliamentary jobs "political assassination," Fillon called on his supporters to "resist" and said he would leave it up to voters to decide his fate.
A defiant Francois Fillon on Wednesday said he will not drop out of the French presidential elections despite allegations of corruption.
"I won't give in, I won't surrender, I won't pull out, I'll fight to the end," Fillon said in a speech in Paris.
The former prime minister has been accused of paying his wife, Penelope, hundreds of thousands of euros of taxpayer's money for work she may not have done.
An investigation into the allegations, which opened in January, plunged Fillon's campaign into crisis and cost him his status as the favourite to win the presidency in the two-round election set for April 23 and May 7.
Fillon has denied wrongdoing and says his wife was paid the money for legitimate work as his parliamentary assistant, though he has acknowledged giving her the work was an error of judgement.
TRT World's Craig Copetas weighs in.
Fillon abruptly postponed his visit to the annual Paris agricultural fair on Wednesday, viewed as a key campaign stop for presidential candidates.
The postponement comes amid a report by French newspaper Journal du Dimanche, that he had been summoned by magistrates investigating allegations he paid family members for fake parliamentary jobs.
Fillon is facing a full judicial inquiry into the so-called "Penelopegate" affair.
The investigation has unnerved investors who fear Fillon's campaign woes have handed the anti-euro, anti-immigration Marine Le Pen of the National Front a higher chance of winning the presidency.
Fillon – who has long cultivated an image of probity and criticised people for taking government handouts – has been heckled for weeks by protesters at campaign outings.